Feature: Param to change Output topic

[austin-InDro]

Nice work on this code!

I think it would be nice to have the ouput mux variable in case there is a new node you intergrate into your system that by default wants to publish to cmd_vel. By having your robot driver set to another topic, it would remove the danger of code taking over from your mux. It seems it manually has to be changed on the following line in your src node.

output_topic_pub_ = this->create_publisher<geometry_msgs::msg::Twist>("cmd_vel", 10);

Thoughts?